Apparatus for reduction drawing of hollow bodies of stainless steel

ABSTRACT

A cup shaped stainless steel blank is deep drawn into a closed end hollow shape having a thickened bottom and a thickened rim at its open end. A ram for forcing the blank through a drawing die has a forward plunger portion and a conical body portion merging with and tapering rearwardly from the plunger portion. Axial forward thrust of the ram forces its plunger portion into the blank and pushes the latter through the drawing die, thereby transforming the blank into an elongated hollow shape having a uniform outside diameter from end to end and an inwardly projecting rim at its open end. Stripping of the elongated hollow shaped from the ram radially distends the inwardly projecting rim so that the stripped hollow shape will have the same inside diameter from end to end.

United States Patent Oberl'ainder et a1.

(75] Inventors: Karl-0skar Oberl'a'nder; Christian Domscha; WernerSchurr, all of Geislingen/Steige, Germany [73] Assignee:Wurttembergische Metallwarenfabrlk, Geislingen/Steige, Germany [22]Filed: Nov. 13, 1973 [21] Appl. No.: 415,312

[30] Foreign Application Priority Data Nov. 16, 1972 Germany 2256334Mar. 29, 1973 Germany 2315832 [52] U.S. C1 72/347; 72/467 [51] Int. Cl321d 22/20 [58] Field of Search 72/347, 348, 349, 467;

[56] References Cited UNITED STATES PATENTS 1,944,527 1/1934 Pfaendler72/347 2,877,546 3/1959 Wilcox 1 72/347 2,882,759 4/1959 Altwicker76/107 A July 8, 1975 3,203,218 8/1965 Bolt et a1. 72 349 3,423,9851/1969 3,466,922 9/1969 3,785,311 1 1974 Yoshikawa 113/120 H FOREIGNPATENTS OR APPLICATIONS 392,192 3/1924 Germany 72/347 PrimaryExaminer-Richard .1. Herbst Attorney, Agent, or Firm-James E. Nilles[57] ABSTRACT A cup shaped stainless steel blank is deep drawn into aclosed end hollow shape having a thickened bottom and a thickened rim atits open end. A ram for forcing the blank through a drawing die has aforward plunger portion and a conical body portion merging with andtapering rearwardly from the plunger portion Axial forward thrust of theram forces its plunger portion into the blank and pushes the latterthrough the drawing die, thereby transforming the blank into anelongated hollow shape having a uniform outside diameter from end to endand an inwardly projecting rim at its open end. Stripping of theelongated hollow shaped from the ram radially distends the inwardlyprojecting rim so that the stripped hollow shape will have the sameinside diameter from end to end.

1 Claim, 7 Drawing Figures mum JUL 81% SHEET 1 APPARATUS FOR REDUCTIONDRAWING OF HOLLOW BODIES OF STAINLESS STEEL BACKGROUND OF THE INVENTIONThis invention relates to a method and apparatus for reducing drawing ofhollow bodies, in particular for the production of cooking utensils, inparticular pressure cookers, comprising a reinforced bootom and edgefrom a cup formed by drawing.

Cooking utensils with a thick bottom a thinner cylindrical wall and areinforced top edge have approximately the same stiffness in both thewall and the bottom. The may, therefore, be subjected to a certaininternal pressure as occurs, for example, during pressure cooking. Thereinforced edge is desirable for securing a cover in place as requiredfor pressure cooking.

According to a heretofore known process of producing hollow shapes, acup is formed by deep drawing and further developed by pressure rolling.In this process, the cup is positioned on a rotating pressure casing.The cup is engaged from the outside by at least one pressure rollerwhich is moved in the axial direction of the mandrel-like pressurecasing. During this production process, it is possible to produce areinforced edge for a cooking utensil. This process, however, isexpensive, since deep drawing and pressure rolling each require aworking operation and special machines. Furthermore, pressure rolling istime-consuming and limited to hollow circular-cylindrical bodies.

SUMMARY OF THE INVENTION The principal object of the present inventionis to provide an improved process and apparatus for producing deep drawnhollow shapes of stainless steel in a simple and economical manner.

This object is accomplished in accordance with the invention bypositioning the cup on a ram and pushing it by means of said ram throughat least one reduction drawing die in a manner known as reductiondrawing, said reduction drawing die forming together with said ram anannular gap whose width is smaller than the thickness of the cup walland by subsequently stripping the elongated blank from said ram. Whilethe wall material of the cup adjacent its free edge region passesthrough the reduction drawing die said wall material is pushed into atapered zone of the ram during the reduction drawing operation beforethe free cup edge passes through the smallest diameter of said drawingdie and the edge region, which has been pressed into said tapered zoneis then expanded to the diameter of the untapered ram as said hollowbody is stripped off said ram.

The reduction drawing technique has heretofore been used for theproduction of hollow bodies comprising a reinforced bottom and a thinnerwall portion, but in the bodies as heretofore produced by said techniquethe wall thickness of the hollow body was uniform over its entirelength. By pressing the cup material into the tapered zone of the ram inaccordance with the present invention, an edge region is produced whichhas a reduced internal diameter and which is thickened toward the insideof the hollow body. During subsequent stripping of the hollow body fromthe ram, the non-tapered section of the ram moves outwardly through thisconstricted region thus expanding it to the normal internal dimension ofthe hollow body. This results in a thickening of the edge region in anoutward direction. Thus,

during the stripping operation, a material deforming operation occurs incontrast to previously known processes. This however, does notnecessitate any additional expenditure of time. The savings in timeachieved with the inventive process as compared to the pressure rollingprocess is considerable. It, of course, depends on the size, inparticular the height, of the cooking utensil and generally lies in arange between 30 and Another advantage of the process according to theinvention is in the apparatus required which necessitates much lessexpenditure than separate deep drawing and pressure rolling means. Inaddition, the inventive process also lends itself, in addition to theproduction of cooking utensils with circular-cylindrical shape, to theproduction of hollow bodies having another cross section, for example,of oval or rectangular shape.

The invention also provides an apparatus for performing the process, andwhich apparatus comprises an axially movable ram and at least onereduction drawing die which, together with said ram, forms an annulargap whose width is smaller than the thickness of the wall of the cupforming the starting workpiece. The apparatus further comprises astripping means, and the ram has a tapered zone in the region in whichthe cup edge comes to rest before it is passed through the smallestdiameter of said reduction drawing die.

The cup edge material is pressured through the reduction drawing dieinto this tapered zone. As soon as the tapered zone has reached thesmallest diameter of the reduction drawing die, the annular gap betweenthe drawing die and the ram becomes gradually wider so that the edgeregion, which has been thickened in an inward direction toward the ram,can pass this position. Another advantage of this arrangement is thatthe forces of pressure present in the reduction drawing ring abateslowly and not suddenly when the cooking utensil emerges from thisposition. In this way, the material of the reduction drawing die is notsubjected to abrupt great differences in pressure.

In order to be able to employ the inventive process in the case ofhollow bodies of stainless steel, the reduction drawing die in a furtherdevelopment of the inventive apparatus consists of a ceramic materialfor the reduction drawing of cups of stainless steel. In previouslyknown processes for reduction drawing with a ram which has a uniformdiameter along the entire length of a region coming into contact withthe material to be reduction-drawn, the result in the case of stainlesssteels, in particular austenitic or ferritic steels, was that coldfusion tended to occur between the steel and the drawing die inConjunction with the tool steel or with the sintered metal of thereduction drawing die, thereby causing the tension forces to increaseconsiderably after a few draws and the resultant hollow bodies were justas useless as the reduction drawing die due to surface damage andgrooves. The use of a special lubricant has reduced the occurrence ofcold fusion between the blank and drawing die, but has nevertheless notproduced any satisfactory surface quality. In addition, it was difficultto remove this lubricant after manufacture which, however, wasabsolutely necessary for subsequent annealing which was required. Testswere also made with reduction drawing dies consisting of ceramicmaterial, but these have hitherto failed due to the fact that the suddenrelease or drop in pressure as the workpiece emerges from the reductiondrawing die during reduction drawing led to fissures in the ceramicmaterial. In the inventive apparatus, the fact that the cup material ispressed into a tapered zone of the ram before the free cup edge is drawnthrough the smallest diameter of the drawing die prevents a suddenrelease or drop in pressure in the material of the drawing die. On thecontrary, the pressure is reduced gradually so that no fissures occur inthe drawing die when a ceramic material is used and this drawing dieachieves a long service life which satisfies economical demands.Reduction drawing dies of ceramic material are advantageous in that nocold fusion between the blank and die occurs and the resultant hollowbodies of stainless steel have a perfect surface. Lubrication andsimultaneous cooling may be accomplished with the help of simplelubricants which can be readily removed subsequently in a washingmachine.

The depth of the tapered zone is advantageously dimensioned such thatthe annular gap is expanded to an amount. at least approximately 80%,preferably 100-12096 of the original cup wall thickness as said taperedzone passes through the smallest diameter of the reduction drawing die.If the depth of the tapered zone produces an expansion of the annulargap to an amount somewhat below the original cup wall thickness, acertain amount of residual pressure tension remains which is suddenlyreduced by passing the cup edge through the smallest diameter of thedrawing die. If this tension only amounts to a fraction of the maximumtension which occurs, however, this does not result in destruction ofthe ceramic material. It is more favorable, however, if the annular gapis expanded by the tapered zone of the ram to an amount which is equalto or somewhat greater than the original cup wall thickness. The cupwall thickness then normally increases somewhat toward the free edge ofthe cup because the cups used as the starting workpiece in reductiondrawing are previously deep-drawn. Dimensioning the tapered zone in thismanner results in a complete, but gradual reduction of pressure tensionas the tapered zone passes through the smallest diameter of the drawingdie.

The tapered zone is advantageously formed by an approximately conicalshoulder on the ram. The resultant reduction in pressure tension, whichprogresses approximately linearly, protects the device. Furthermore, agradual transition between the wall and the thickened edge region isalso produced on the manufactured cooking utensil on the outer side.This is more advantageous than a sharp transition, both with respect tofurther manufacturing processes, for example, during bending over of theedge and during enameling, as well as during later use, in particularcleaning of the cooking utensil.

The entrance angle of the shoulder should amount to betweenapproximately 6 to 30. The size of the entrance angle is decisive forthe height of the thickened edge region on the finished cooking utensil.The smaller the entrance angle, the higher the edge region. A smallerentrance angle, however, also leads to smaller forces present when thecooking utensil is stripped from the ram because the material, which hasbeen pressed into the tapered zone, does not have to be repushedoutwardly as far. On the other hand, a greater entrance angle producesan edge thickening of the finished hollow body which is not as high asis normally the case. Greater stripping forces, however, must be exertedin this case. Depending on which of these points 4 predominates, theentrance angle can be selected toward the upper or lower limit.

In known apparatus for deep drawing hollow shapes, stripping means arenormally used which are designed like fingers. In the improved apparatusaccording to the present invention, the stripping means are designed asasplit ring whose internal diameter corresponds to the external diameterof the untapered ram. The stripping means thus have a considerablygreater abutment surface on the cooking utensil which is to be strippedoff the ram. This enlargement in the abutment surface is favorablebecause relatively great forces must still be overcome due to thedeformation of the edge region when the hollow body is stripped off theram.

In yet another embodiment of the present invention. the stripping meansinclude an ejector which can be moved axially with respect to the ram.This ejector can engage practically the entire inner bottom surface ofthe cooking utensil to be stripped off the ram. The distribution offorce and thus the tension ratio occurring in the cooking utensil areespecially favorable in this case.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects andadvantages of the invention will become more fully apparent as thisdisclosure proceeds with reference to the accompanying drawings inwhich:

FIG. 1 is a somewhat schematic vertical section of an apparatus for deepdrawing hollow shapes, embodying the invention;

FIGS. 2-6 are views similar to FIG. 1 illustrating various steps of amanufacturing process incorporating the invention, FIG. 6 alsoillustrating a modified stripping device which may be used in theapparatus shown in FIG. I; and

FIG. 7 is a sectional elevation of the body of a cooking utensilproduced in accordance with the invention.

DETAILED DESCRIPTION In the drawings, I indicates a stationary reductiondrawing die which is mounted in a receiving ring 2 with a shrink fitunder pressure. The reduction drawing die consists of a ceramicmaterial, preferably high purity M 0 sintered corundum.

The drawing die 1 has an inlet surface la (FIG. 6) which tapersconically in the direction of drawing and whose entrance angle may liebetween 8 and 0. A cylindrical section lb whose height is approximately3-5 mm follows the inlet surface 1a. This cylindrical section lb isitself followed by an outlet surface lc which flares conically in thedirection of drawing and whose entrance angle may amount toapproximately 345.

The cylindrical section lb (FIG. 6) defines the working edgediameter dof the reduction drawing die I (FIG. 1).

A ram which is generally designated as 3 cooperates with the drawing die1 and includes a cylindrical for- ,ward plunger portion 3a, a stemportion 3b which is ,should have an entrance angle of approximately 6 to30. The ram 3 is movable in the direction of the double headed arrow Prelative to the drawing die 1. During this movement through said drawingdie, an annular gap is formed between the plunger portion 3a and thedrawing die 1 with the smallest width calculated from the difference ofthe diameter d and the diameter s of the plunger portion 3a of the ram3. The width of this annular gap is designated as r in FIG. 1.

A stripping means is disposed at the outlet side of the drawing ring 1.In FIGS. 1 to 5 and in the right half of FIG. 6, the stripping meansconsists of a ring assembly 4 comprising two or more arc segments whichare urged by a circular spring 5 into the innermost position shown inFIGS. 1, 5 and 6. In this position, the inside diameter of the ringassembly 4 corresponds substantially to the outside diameter 5 of theforward plunger portion 3a of the ram. The ring assembly 4 has aconically crowned inlet surface 4a flaring rearward toward the drawingring 1.

The left hand of FIG. 6 shows an embodiment of the invention withanother stripping means. An ejector '7 is reciprocably mounted in theram 3 by means of bearing elements 8 so as to be axially displaceable.FIG. 6 shows this ejector in the discharge position, i.e., abutting thebottom of the cup 6.

The mode of operation of the apparatus shown in FIG. 1 is as follows: Acup-shaped blank 6 of stainless steel is slipped over the ram 3. Thismay be done by positioning the cup on the inlet surface 10 of thereduction drawing die 1 so as to be centered thereon The ram 3 may thenbe pushed into said cup 6.

The cup shaped blank 6 may conventionally be manufactured from acircular disc of sheet metal by backward extrusion, the blank havingsubstantially the same thickness at its side wall and base. The wallthickness may be somewhat expanded towards the free edge 60 as a resultof the deep drawing operation. The starting position with the cup 6seated on the forward cylindrical plunger portion 3a of the ram is shownin FIG. 1.

By thrusting the cylindrical plunger portion 3a of the ram 3 through thereduction drawing die 1, the material of the cup 6 initially comes torest on the inlet surface la and is then pushed into the cylindricalsection lb (FIG. 2) of the drawing die 1. The wall material of the cup 6is then reduction-drawn to the gap width r (FIG. 1) between thecylindrical plunger portion 3a of the ram 3 and the cylindrical sectionlb of the drawing die 1. In this manner, reduction drawing up to 40% isreadily possible within the scope of the invention. The reductiondrawing of the wall material from the original wall thickness of the cup6 down to the thickness r is best illustrated by FIG. 2.

During the reduction drawing operation, a simple lubricant, for example,soap water, which cools as well as lubricates the drawing die 1, may besprayed out of a nozzle indicated at 7 in FIG. 2.

As the ram 3 continues to push through the drawing die I, the wallmaterial of the cup 6 is extended along the ram. In FIG. 2, the free cupedge 60 has already reached the tapered shoulder presented by theconical body portion 3c. This is disposed at such a location on the ram3 that the free cup edge 60 still has a clear axially spaced relationwith respect to the smallest diameter of the drawing die I when itarrives at the shoulder 30.

The conical body portion 3c defines a tapered zone V around the ram 3(FIG. 3). As soon as this zone V, i.e., the conical ram portion 3c,reaches the area of the smallest diameter d of the drawing die 1, thegap between the ram 3 and the drawing die 1 enlarges radially from anamount r (FIG. 1) to an amount which corresponds to the enlargement ofthe tapered zone V. The tapered zone V is dimensioned such that thewidth of the gap between the cylindrical section of t he drawing die 1and the rear cylindrical section 3b of the lam at the trailing end ofthe conical body portion 30, approaches the wall thickness of theoriginal cup 6 and preferably exceeds it by 10-20%.

FIGS. 3 and 4 show that the cylindrical section lb of the drawing die 1urges the wall material of the cup 6 against the conical body portion 30of the ram so that a compacted constricted edge region 6b of the cup isformed which still has a completely cylindrical external wall surface atthis time.

It is an essential feature of the invention that the ma-' terial of theedge region of the cup 6 be pressed into the tapered zone around the ram3 before the free edge 6a passes through the smallest diamater d of thedrawing die 1. This condition is met in the embodiment shown in thedrawing. One can see in FIG. 4 that the compacted edge region 612 fillsout the tapered zone around the ram.3 whereas the free edge 6a of thecup is just moving into the cylindrical section lb of the drawing die 1.

The ram 3 then pushes the reduction-drawn cup 6' against the inletsurface 4a of the stripper ring assembly 4 so that this assembly isclistending radially. As soon as the free edge of the reductiondrawn cup6" comes to lie below the stripper 4 (FIG. 5), the stripper sectors snapback in an inward direction due to tlfe action of the spring 5 and theycome to lie above the free edge of the drawn cup 6.

When the ram 3 is withdrawn in an upward direction, the cup 6 cannot bepulled along because its free edge 6a is stopped by the underside of thestripper 4. The cup 6' therefore slides off the ram. In so doing, thecompacted reinforced edge region 612, which abuts the conical ramportion 3c and the rear cylindrical ram portion 3b, is radially expandedby tlte forward cylindrical plunger portion 3a of the ram. This is shownin the right half of FIG. 6.

The mode of operation of the ejector 7 may be seen in the left half ofFIG. 6. The ejector pushes against the bottom of the cup 6' while theram 3' is withdrawn. The edge region of the cup 6' is expanded inexactly the same manner as before, but is subjected to pull whereas itis subjected to push in the embodiment according to the right side ofFIG. 6.

The final result of the reduction drawing process is a cylindricalhollow body with a reinforced wall region 6'b and a base 6'6 ofapproximately the same thickness. The inner wall is completelycylindrical (FIG. 7). The cylindrical wall 6d has a uniform thicknesscorresponding to the gap width r. In the next operational step in theproduction of a cooking utensil, the rein forced edge is bent outwardly.It is nickel,and chlzome.

tapered body portion 3c and the entrance angles of the drawing die 1shown in the drawing may be varied within the limits statedhereinbefore.

Two or more drawing dies may be coaxially disposed relative to oneanother and may be brought successively into action instead of a singledrawing die I. The first drawing die may be disposed downstream inrelation to a drawing ring in which the cup 6 forming the starting blankis drawn from a round disc of sheet metal.

The drawing die may consist of cemented carbide or tool steel whenproducing hollow bodies from normal deep-drawing steels or non-ferrousmetals.

The operation of forcing the ram 3 axially into the cup shaped blank 6so as to advance the latter through the darwing die assembly transformsthe blank into an elongated body as sohwn in FIG. which has a bottomportion of substantially the same thickness as the bottom thickness ofthe blank, a peripheral wall portion of less thickness than theperipheral wall of the blank, and an inwardly projecting edge portionadjacent the open end of the hollow body. Stripping of the hollow bodyfrom the ram causes radial expansion or distension of its inwardlyprojecting edge portion as illustrated by FIG. 6, and may beaccomplished either by pulling the ram through the open end of thehollow body, or by subjecting the hollow body to an internal axialexpanding force between its closed end and the ram. In the latter case.the axial expanding force may be produced by an ejector element. such asthe ejector 7 shown in FIG. 6, which is reciprocably movable on the ramin front of its forward plunger portion.

We claim:

I. In an apparatus for producing hollow bodies of stainless steel. thecombination of an axially reciproca ble ram and a surrounding dieassembly cooperable with said ram so as to reduction deep draw acupshaped stainless steel blank. said die assembly comprising astationary support and an annular die member of ceramic material fixedlymounted thereon and said ram having a forward plunger portion and anattenuated axial portion tapering rearwardly from said forward portion,the peripheral exterior surface of said ram being cooperable with theperipheral interior surface of said blank. and the peripheral exteriorsurface of said blank being cooperable with the inner edge of saidceramic die member so that by an initial forward movement of said ram aperipheral wall portion of said blank at its closed end will be drawn toan axially extended length of radially reduced wall thickness and sothat continued forward movement of said ram will deform the free end ofsaid blank into a rearwardly extending annular rim of inwardlyincreasing radial thickness, thereby gradually reducing the radialoutward load upon said ceramic die member; and stripping means foraxially withdrawing said extended and deformed blank forwardly from saidram.

1. In an apparatus for producing hollow bodies of stainless steel, thecombination of an axially reciprocable ram and a surrounding dieassembly cooperable with said ram so as to reduction deep draw acup-shaped stainless steel blank, said die assembly comprising astationary support and an annular die member of ceramic material fixedlymounted thereon, and said ram having a forward plunger portion and anattenuated axial portion tapering rearwardly from said forward portion,the peripheral exterior surface of said ram being cooperable with theperipheral interior surface of said blank, and the peripheral exteriorsurface of said blank being cooperable with the inner edge of saidceramic die member so that by an initial forward movement of said ram aperipheral wall portion of said blank at its closed end will be drawn toan axially extended length of radially reduced wall thickness and sothat continued forward movement of said ram will deform the free end ofsaid blank into a rearwardly extending annular rim of inwardlyincreasing radial thickness, thereby gradually reducing the radialoutward load upon said ceramic die member; and stripping means foraxially withdrawing said extended and deformed blank forwardly from saidram.